Sonar signalling techniques for underwater speed measurement and echo location and other related techniques require precise determination of the delay between the time at which an acoustical signal pulse train is propagated into the water and the time at which the signal (or a relection of the signal) is received. Moreover, multiple path effects require that the transmitted acoustical pulses have a very short time duration. However, because the pulse energy decreases with the pulse duration, a compromise must be made between the degree of accuracy to which the pulse propagation delay can be ascertained and the signal-to-noise ratio which may be attained.
One way of overcoming the foregoing limitation is to pseudo-random noise code the transmitted acoustical pulses using either binary amplitude or phase modulation techniques. The received signal may then be decoded by correlating it, in known fashion, with a selected portion of the transmitted pseudo-random noise coded signal. The present invention provides a method and apparatus for utilizing the phase component of the received signal to determine the signal propagation delay time with great percision.